The present invention relates generally to methods for preparing inorganic oxides and relates more particularly to a method for preparing titanium oxide precipitates of desired morphologies.
Inorganic oxides are currently being used in a number of diverse commercial and military applications, such as in the fields of photovoltaics, photocatalysis, enzyme support, energy storage, sensors, pigments, and photonics. By way of example, one type of recently developed photovoltaic system utilizes photosensitive dyes on the surfaces of titanium oxide (i.e., titania) films. As another example, metal oxides have found use as sensors, where the sensing agent is entrapped within a metal oxide matrix (see, for example, Chen et al., Chem. Mater., 18(2): 5326-35 (2006), which is incorporated herein by reference). As yet another example, metal oxides have found use in military settings for laser eye protection and shielding. As a further example, some metal oxides, such as aluminum oxide and titanium oxide, have been used to decontaminate hazardous chemicals in applications ranging from water purification to protection against chemical warfare agents.
For many of the above-described applications, the morphology of the inorganic oxide material may have some bearing on its usefulness for its intended purpose. For example, in the case of titanium oxide in photovoltaics, it has been found that titanium oxide nanorods, with their greater surface areas, give greater energy yields than bulk titanium oxide films (see, for example, Limmer et al., Adv. Funct. Mater., 12(1):59-64 (2002), which is incorporated herein by reference); however, current methods of producing such nanorods are difficult and expensive.
The idea of preparing titanium oxide having a particular morphology is not a new concept. The traditional approach to obtaining titanium oxide having a desired morphology has been to precipitate the titanium oxide onto a template having the desired shape (see, for example, Imhof, Langmuir, 17:3579-85 (2001), and Li et al., Langmuir, 24(19): 10552-6 (2008), which are incorporated herein by reference). Polymers are one of the most widely used types of materials used as a template. This is largely due to their controllable size and shape, straightforward chemical modification, and ease of removal with heat or solvent after precipitation. Poly(methylmethacrylate) (PMMA) and poly(styrene) are two polymers that have commonly been used as templates and have been used to create titanium oxides of certain shapes, such as core/shell composites, hollow shells, tubes, and 3-D porous structures. Most of the methods for titanium oxide formation that use a template involve a sol-gel procedure, in which, after precipitation, the template is removed using heat or a harsh solvent.
More exotic morphologies than those obtainable using the aforementioned polymeric templates occur naturally in nature, and there have been some gas and solution phase studies to determine the feasibility of using these biomaterials for templating. For example, diatoms are unicellular organisms that create thousands of different 3-D morphologies of silicon oxide in their exoskeletons for environmental protection (see, for example, Cha et al., Nature, 403:289-292 (2000), and Sumper et al., J. Mater. Chem., 14:2059-65 (2004), which are incorporated herein by reference). The exoskeletons of these organisms can be modified using gas/solid displacement to incorporate other atoms in place of silicon in their structures. This approach preserves the integrity of the exoskeleton morphology, which can then be integrated into electronic devices. As another example, sea urchin plates have been coated with silicon oxide and titanium oxide using a sol-gel method, in which the template can thereafter be dissolved away with a strong acid. As yet another example, zinc oxide has been shaped using butterfly wings as a template (see, for example, Zhang et al., Bioinsp. Biomim., 1:89-95 (2006), which is incorporated herein by reference). The wing is dipped into an ethanol/water solution of zinc nitrate and treated with high temperature to form a zinc oxide “wing.” The actual butterfly wing is then ashed with heat, leaving the “duplicate” zinc oxide wing. Although the above-described use of biological templates enables certain morphologies not otherwise attainable to be achieved, these approaches are still limited by the available shapes of the biological template, as well as being limited by long-term commercial applicability. Finally, all of the above approaches are limited in the sense that they cannot also concurrently incorporate environmentally sensitive materials during oxide formation, due to the heat and/or harsh solvent typically used during the process. This drawback further limits their potential multifunctionality.
Recent mechanistic studies on naturally occurring biomolecules, such as silaffin (peptide) and silicatein (enzyme), have led to novel biomimetic approaches to inorganic oxide formation. The initiating materials for these studies were chosen in order to mimic the active components of the natural biomolecules. These methods have mostly utilized either a small molecule or a polymer containing hydroxyl and/or amine side chains to initiate precipitation of different oxides from solution (see, for example, Sewell et al., Chem. Mater., 18:3108-13 (2006), and Gorna et al., Macromol. Biosci., 7(2):163-73 (2007), which are incorporated herein by reference). Most of these studies only report on whether the material has the ability to precipitate the oxide, without a discussion of morphological control. No system to-date has demonstrated an ability to create multiple different morphologies without changes in reaction conditions, such as reaction temperature, buffer composition and pH.